The Scientific Basis for the Teaching and Practice of Conservative Operative Dentistry

Once a carious lesion requires operative intervention to halt the caries process and restore lost tooth structure, what form should that intervention take and what factors are involved in providing maximum longevity of the resulting restoration and tooth? This paper looks at the evidence for conservative operative intervention, attempts to assess the relationship between cavity preparation and restoration survival, and documents the major factors involved in restoration failure.

Conservative Cavity Preparation

Traditional operative dentistry involves standardized preparation that utilizes differing degrees of "convenience form" (access to caries) and "extension for prevention" (placing cavity margins in less caries-susceptible locations) and can reduce the structural and biological integrity of teeth. Conservative forms of operative intervention have now been recommended that concentrate more specifically on removal of carious dentine and preservation of as much sound tooth structure as possible. These are discussed below.

The Proximal "Tunnel" Restoration

The "tunnel" concept, which accesses proximal dentinal caries through a sound mesial or distal occlusal pit to preserve the proximal marginal ridge, was described by Hunt (1984). A total of 10 clinical trials in permanent teeth and 2 in primary teeth on this concept had been conducted through the 1990s. Early clinical reports utilized small numbers of glass ionomer restorations and indicated that the technique was promising. but later reports found higher failure rates. Use of a metal cermet glass ionomer gave little evidence of inhibition of recurrent caries, while the most frequent causes of restoration failure were marginal ridge fracture and recurrent decay. A higher proportion of the marginal ridge fractures was associated with more extensive tunnel preparations. The longest clinical study (7 years) reported a 50 percent survival time of 6 years for restorations, (Hasselrot, 1998), while two recent multi-operator trials provided evidence of high rates of associated caries (41-45 percent) as early as 3 years (Nordbo, Leiskar, von der Fehr,1998; Pilebro, van Dijken, Stenberg, 1999). Poor performance in primary teeth has also been documented (Hasselrot, 1993; de Freitas, de Andrada, Baratieri, 1994).

Many studies of the tunnel concept utilizing baseline radiographs have reported evidence of inadequate caries removal (Hasselrot, 1993, 1998; Strand, Nordbo, Tveit, 1996; Pilebro, van Dijken, Stenberg, 1999). This was presumably due to the blind approach provided by limited access. Visibility was only improved by enlarging the occlusal access, thus reducing the conservative nature of the technique (Knight, 1992). Low restoration survival was associated with limited preparation-extension in high caries individuals, especially where demineralized proximal enamel was left in order to avoid cavitation of the proximal surface (Strand, Nordbo, Tveit, et al., 1996; Pilebro, van Dijken, Stenberg, 1999).

This technique is limited to treatment of early dentinal decay, often prior to enamel cavitation. Since cavitation is becoming accepted as the stage that defines the necessity for operative intervention, the technique has limited use. The low effectiveness reported argues in favor of a more direct approach to proximal caries. It also affirms the difficulty in arresting proximal caries.

The Proximal "Box-Only" Restoration

Traditional Class 2 cavity preparation for the treatment of proximal caries involves both a proximal and occlusal portion of the tooth. Changes in this approach have been recommended where only the proximal tooth structure is carious. Although "box-only" (or "slot") preparations for amalgam were introduced in 1973 (Almquist and colleagues) and "adhesive slot" preparations for resin composite were introduced in 1978 (Simonsen), such conservative restorations are still relatively rare in general dental practice. Our search of the literature turned up only three clinical studies of these kinds of restorations in permanent teeth. One study found no failures in 68 composite box-only restorations over 5 years (Kreulen, Tobi, van Amerongen, 1998). Another found that the 10-year success rate for composite proximal "saucer" preparations was 68.6 percent (Nordbo, Leiskar, von der Fehr, 1998). Half of the failures were due to recurrent decay, and half were considered technique-related. Recurrent caries, when present, occurred only at the gingival margin, not bucco-lingually, justifying the minimal lateral and occlusal extension. Loss of retention did not occur. A third study found no failures in amalgam restorations of this kind over periods of 5 to 7 years (Lumley, Fisher, 1995). All three trials give us good evidence that the proximal slot-only restoration is a viable treatment option, providing similar or better longevity compared to conventional Class 2 composite or amalgam restorations, and greater tooth preservation. In short, the technique was reported to be superior to tunnel restorations, probably because of better operator visibility.

Four studies of modified proximal restorations in primary teeth involved were found, of which three were of only 1-year duration. The fourth and longest (3 years) showed poor performance for a cermet glass ionomer but significant improvement with a resin-modified glass ionomer, with an estimated median survival time exceeding 42 months (Espelid, Tveit, Tornes, et al., 1999).

Gingival Margin Location

Gingival extension of Class 2 restorations, whether traditional or box-only design, is of particular importance. Most recurrent decay occurs in the gingival proximal location (Mjor, 1998; Klausner, Green, Charbeneau, 1987). The "extension for prevention" concept suggests that subgingival margins reduce the risk of secondary caries, but the evidence for this comes from the prefluoride era. The need for appropriate location of the gingival proximal margin was shown to be important in a rare clinical trial that examined the relationship between proximal cavity design and recurrent caries (Otto, Rule, 1988). Restorations with gingival margins that did not clear the contact area had a significantly higher rate of caries at all time intervals over a 2-year period. Since creating a "self-cleansing" location for the gingival margin of proximal restorations is impossible, good home care by patients is essential. Whether conservative gingival extension increases the risk of recurrent caries in the absence of such home care remains to be determined.

The Preventive Resin Restoration

The preventive resin restoration (PRR) is a conservative occlusal restoration that involves replacement of discrete areas of carious tooth structure with composite, followed by application of an overlying fissure sealant, instead of the traditional "extension for prevention" (Simonsen, 1980).

A total of 18 clinical studies on the PRR were published between 1978 and 1999. Although they report generally favorable outcomes, all 18 also report the loss of all or a portion of the sealant as a major problem. The success rates of the studies are not easily comparable, since definitions of failure were variously reported as presence of actual caries or loss of sealant. Three of the studies involved a direct comparison of PRR with silver amalgam (Azhdari, Sveen, Buonocore, 1979; Welbury, Walls, Murray, et al., 1990; Cloyd, Gilpatrick, Moore, 1997). The PRR was at least as successful as amalgam in two of the trials for a period up to 5 years, with the added advantage of preservation of sound tooth structure, but Cloyd and colleagues found sealant failure to be a significant problem, leading to recurrent caries in 8.1 percent of patients. No amalgam failures were recorded over 3 years. None of the 18 studies found occlusal caries when the sealant remained intact, though many did not utilize radiographs at recall. All cases of occlusal caries (up to 24 percent after 9 years) were associated with sealant failure, but the incidence of sealant failure was significantly higher than the presence of caries (Houpt, Fukus, Eidelman, 1994). Loss of sealant over glass ionomer restorative materials (Gray, Paterson, 1994; Kilpatrick, Murray, McCabe, 1996) and larger areas of composite restoration (Gray, 1999) was high. Another study (Mertz-Fairhurst, Curtis, Ergle, et al., 1998) found that sealed composite restorations were able to halt the radiographically observed progress of frank carious dentin over a period of 10 years. This provides some reassurance in cases of inadvertent sealing of incipient dentinal caries and has implications for the conservative treatment of deep carious dentine in the vicinity of the pulp.

In summary, PRR is a predictable and effective conservative treatment for localized areas of occlusal decay, with longevity dependent on retention of the overlying sealant.

Factors Involved in Restoration Failure

Secondary caries is the most frequently cited reason for restoration failure or replacement, followed by fractured restorations. The reasons for replacement are related to many clinical variables that have been grouped as either patient, operator, or dental material factors. A systematic review of dental restoration longevity (Downer, Azli, Bedi, et al., 1999) found strong indications of both patient (age and caries activity) and operator factors. High caries activity in relation to bacterial assay and salivary flow rates (Bentley, Broderius, Drake, et al., 1990; K�hler, Rasmussen, Odman, 2000), poor oral hygiene and PI scores (Goldberg, Tanzer, Munster, et al., 1981; Eriksen, Biertness, Hansen, 1986) and incidence of new primary or secondary caries (Jokstad, Mjor, 1991a and b) are all common reasons for restoration replacement.

The frequency of restoration replacement is higher in younger populations, and highest in the primary dentition (Wendt, Koch, Birkhed, 1998). Both recurrent caries and failure of materials figure prominently in primary dentition studies. Whereas there is some evidence for caries susceptibility as a factor in primary restoration failure, there is also strong evidence that age at time of treatment and size of the restoration are factors (Wong, Day, 1990). Problems with materials are pronounced, with survival times longest for stainless steel crowns and shortest for conventional glass ionomer restorations in posterior teeth (Papathanasiou, Curzon, Fairpo, 1994; Kilpatrick, 1993).

While materials and operator skill are important factors in recurrent caries, the problems seem to be more closely related to patient management of tooth care.